Fluoro-substituted europium activated gadolinium,yttrium and lanthanum oxides



1968 w. WANMAKER ETAL 3,415,757

FLUORO-SUBSTITUTED EUROPIUM ACTIVATED GADOLINIUM, YTTRIUM AND LANTHANUMOXIDES Filed Aug 5, 1965 INVENTORS' WILLEM L. WANMAKERf ALFRED an.

AGENT United States Patent 3,415,757 FLUORO-SUBSTITUTED EUROPIUMACTIVATED GADOLINIUM, YTTRIUM AND LANTHANUM OXIDES WillemLatnhertusWanmaker and Alfred Bril, Emma- 'singel, Eindhoven, Netherlands,assignors to North American Philips Company, Inc., New York, N.Y., acorporation of Delaware Filed Aug. 5, 1965, Ser. No. 477,495 Claimspriority, application Netherlands, Aug. 7, 1964,

3 Claims. (Cl. 252-3014) ABSTRACT OF THE DISCLOSURE Red luminescentphosphor for color television cathode ray tubes consisting of aneuropium activated oxide of gadolinium, yttrium or lanthanum wherein asmall portion of the oxygen is replaced by fluorine.

The invention relates to a luminescent screen, particularly for use in acathode-ray tube for the reproduction of colour pictures, to a radiationsource comprising such a screen and to red-luminescent substance for usein such a screen.

For many purposes there is desirable :a luminescent substance whichproduces red light upon excitation by electromagnetic rays or bycorpuscular particles e.g. electrons. Electromagnetic radiation isemitted for instance by a mercury vapour discharge either oflow-pressure nature or of high-pressure nature. Since the emittedmercury spectrum contains only a small amount of red rays, it isdesirable to improve the colour rendition obtainable by such dischargelamps by converting part of the emitted ultraviolet radiation into redlight.

For the reproduction of colour pictures by means of a cathode-ray tubeuse is frequently made of a luminescent screen having three luminescentsubstances which emit, upon excitation by electrons, blue, green and redlight respectively. In order to obtain a bright image it is necessaryfor the luminescent substance to have a high yield. For the blue andgreen components there is a wide range of substances fulfilling thisrequirement, but for the red component the choice is very restricted.One of the most frequently used substances is silver-activated zinccadmium sulphide. A further, often employed substance is zincorthophosphate activated with manganese.

An international agreement has been made to have the colour point of thered component correspond to the wavelength of 611 mu. Although theaforesaid redluminescent substances emit, at this wavelength, anadequate amount of light, they have the disadvantage of having anemission spectrum with a broad red band, so that a considerable quantityof energy of the excitation radiation is converted into radiation withwavelengths beyond 661 mu. As a result the lumen-equivalent is low.

A further point of importance 'for luminescent sub stances for use incolour reproduction tubes is the so-called weigh of a lumen, which isproportional to l/y, wherein y is the ordinate of the colour point inthe internationally fixed xyz-colour diagram. For the known redphosphors this weight is unfavourable owing to the high y-value.

A luminescent screen according to the invention is characterized in thatit contains a red-luminescent substance consisting of an oxyfluoride ofone or more of the elements gadolinium, yttrium or lanthanum with afluorine content lying between 0.1 and 5.0% by weight activated "Icewith 0.005 to 0.20 atom of europium per mol of oxyfluoride.

The oxyfluorides of gadolinum, yttrium and lanthanum emit, uponactivation with europium light with very narrow bands in the red portionof the spectrum by excitation by electrons and ultraviolet rays, andparticularly strong lines at 6 12 and 614 mu, that is to say very nearthe aforesaid, standardized Wavelength for the red component ofreproduction screens of colour television tubes.

The lumen equivalent of the europium-activated oxyfluorides ofgadolinium, yttrium and lanthanum is particularly high, that is to sayabout 300 lumen/w. The known, aforesaid, silver-activated cadmiumsulphide has a lumen equivalent of not more than lumen/w. and themanganese-activated zinc orthophosphate lumen/ w. The light yield liesbetween that of the two lastmentioned substances.

With respect to the europium-activated oxides of gadolinium, yttrium andlanthanum, not containing fluorine, a higher quantum yield is obtainedupon the excitation by electrons. The highest quantum yields areobtained with a fluorine content lying between 0.2 and 1.0% by Weight offluorine and an europium content lying between 0.04 and 0.20 atom/moloxyfluoride.

The higher europium contents are to be preferred, since the emittedradiation becomes much redder without a drop in yield.

The use of fluorine has the further advantage that it is not necessaryin the manufacture of luminescent oxyfluorides to use high heatingtemperatures. This is, in general, important, but particularly for thesesubstances, since they display a point of transition from one phase tothe other at a temperature of about 1300 C.

The oxyfluorides according to the invention can be manufactured in -asimple manner by means of a mixture of the oxides of one or more of theelements gadolinium, yttrium or lanthanum with europium oxide and asuitably chosen quantity of ammonium fluoride. It is preferred, however,to introduce the europium in the form of europium fluoride and to addsuch a quantity of ammonium fluoride that after heating the luminescentsubstance at a temperature of about 1200" C. the correct quantity offluorine is contained in the substance. It should then be consideredthat part of the fluorine disappears from the reaction mixture.

The invention will be described with reference to a table. This tableindicates the molecular and weight composition of a number of startingmixtures from which the red-luminescent oxyfluorides according to theinvention are obtained by heating at 1200 C. in air for two hours. Thereis furthermore indicated the luminous yield at room temperature uponexcitation by ultraviolet radiation of wavelength of 253.7 m and thequantum yield q upon excitation by electrons.

The luminous yield (L.0.) at room temperature is measured as a relativevalue with respect to a mixture of antimonyand manganese-activatedhalophosphate and such a quantity of calcium carbonate that the lightoutput was reduced to 54% of the initial light output of the cal ciumhalophosphate.

Apart from the aforesaid favourable properties, the oxyfluoridesaccording to the invention have a satisfactory lumen maintenance withrespect to temperature, which means that even at high temperatures thequantity of emitted red radiation is little smaller than the quantity ofradiation emitted at room temperature. It is therefore possible to loadthe luminescent screens to a high value, which may be the case in colourtelevision display tubes, for projection purposes, and for use in ared-correcting layer in a high-pressure mercury-vapour discharge lamp.

TABLE GdaOa EllFa NH4F EuzOa YzOa L.Otat room emp.

M01 Gins M01 Gms M01 Gms M01 Gms. M01 Gms q The drawing illustrates acathode-ray tube composed of the conical part 1 and the window part 2.On the interior side of this window a luminescent screen 3 according tothe invention has been applied.

What is claimed is:

1. A red-luminescent substance particularly adapted for use in a cathoderay tube for the reproduction of color images, said red-luminescentsubstance consisting of a fluoro substituted oxide of at least oneelement selected from the group consisting of gadolinium, yttrium andlanthanum, having a fluorine content of between 0.1 and 5% by weight andactivated by 0.005 to 0.20 atom of europium per mol of oxide.

15 the fluorine content lies between 0.2 and 1.0% by weight.

References Cited UNITED STATES PATENTS 3,250,722 5/1966 Borchardt252301.4

TOBIAS E. LEVOW, Primary Examiner.

ROBERT D. EDMONDS, Assistant Examiner.

US. Cl. X.R.

2. The red-luminescent substance of claim 1 wherein 25 9

